Add benchmark

[pboling]

• [x] benchmark will no longer be in the Ruby std lib as of Ruby 3.5, so it must be added as an explicit dependency

• [x] Upgrade to new RuboCop plugins architecture (v1.72+)

• [x] Add rubocop-rake, recommended rubocop plugin

• [x] Setup rubocop_todo.yml

• [x] Remove unused RUBY_VERSION conditions, now that minimum supported ruby version is 2.7

• Fixes #48